The invention relates to microswitches and microrelays and specifically to a method for preparing the contacts in these devices so that they work reliably for many (typically a billion or more) cycles.
The making and using of certain types of microswitches and microrelays is generally known. Micromechanical relays are receiving increased attention recently as our community begins to realize the benefits of integration of micromechanical structures with electronics. Development of these devices is being stimulated by a continuing need for small switches with very large ratios of off-impedance to on-impedance. Low on-state resistances are achieved by bringing two conductors into physical contact; high off-state impedances are a result of using small contact areas to minimize capacitance. Examples of such microfabricated switching devices employing electrostatic (P. M. Zavracky, S. Majumder, and N. E. McGruer, “Micromechanical Switches Fabricated Using Nickel Surface Micromachining,” J. Microelectromechanical Systems, Vol. 6, 3-9 (1997); J. Drake, H. Jerman, B. Lutze and M. Stuber, “An electrostatically actuated micro-relay,” Transducers '95 Eurosensors IX, Stockholm, Sweden (1995); M. Gretillat, P. Thiebaud, C. Linder and N. de Rooij, “Integrated circuit compatible electrostatic polysilicon microrelays,” J. Micromech. Microeng. 5 156-60 (1995); K. E. Petersen, “Micromechanical membrane switches on silicon,” IBM J. Res. Dev. 23 376-85 (1979); J. J. Yao and M. F. Chang, “A Surface Micromachined Miniature Switch for Telecommunications Applications with Signal Frequencies from DC up to 4 GHz,” Proc. Transducers '95, Stockholm Sweden, vol. 2, pp384-387, 1995; K. Petersen, “Dynamic Micromechanics on Silicon: Techniques and Devices,” IEEE Trans. On Electron Devices, vol. ED-25, pp. 1241-1250, 1978; J. Randall, C. Goldsmith, D. Denniston, and T-H. Lin, “Fabrication of Micromechanical Switches for Routing Radio Frequency Signals,” J. Vac. Sci. Technol. B, vol. 14, p. 3692, 1996; M. A. Gretillat, P. Thieubaud, C. Linder, and N. F. de Rooij, J. Micromech. Microeng., vol 5, pp 156-160, 1995; J. Drake, H. Jerman, B. Lutze and M. Stuber, “An electrostatically actuated micro-relay,” Transducers '95 Eurosensors IX, Stockholm, Sweden (1995); M. Sakata, “An electrostatic microactuator for electro-mechanical relay,” Proc IEEE MEMS Workshop '89 (Salt Lake City, Utah) 149-51 (1989); S. Roy and M. Mehregany, “Fabrication of Electrostatic Nickel Microrelays by Nickel Surface Micromachining,” Proc. IEEE Microelectromechanical Systems Workshop, Amsterdam, the Netherlands, pp. 353-357, 1995; and I. Schiele, J. Huber, C. Evers, B. Hillerich, and F. Kozlowski, “Micromechanical Relay with Electrostatic Actuation,” Proc. Transducers '97, Chicago, vol. 2., p. 1165, 1997), magnetic (H. Hosaka, H. Kuwano, and K. Yanagisawa, “Electromagnetic Microrelays: Concepts and Fundamental Characteristics,” Sensors and Actuators A, vol. 40, p. 41, 1994; and W. P. Taylor, M. G. Allen, and C. R. Dauwalter, “A Fully Integrated Magnetically Actuated Micromachined Relay,” Proc. 1996 Solid State Sensor and Actuator Workshop, Hilton Head, pp. 231-234, 1996) and thermal (J. Simon, S. Saffer, and C. J. (CJ) Kim, J. Microelectromech. Sys., vol. 6, pp. 208-216, 1997; E. Hashimoto, H. Tanaka, Y. Suzuki, Y. Uenishi, and A. Watabe, “Thermally Controlled Magnetization Actuator for Microrelays,” IEICE Trans. Electron., vol E80-C, p. 239, 1997; and J. Simon, S. Saffer, and Chang-Jin (CJ) Kim, “A Liquid-Filled Microrelay with a Moving Mercury Microdrop, J. Microelectromechanical Sys., Vol 6, p 208, 1997) actuation have been reported. The ideal actuation method would operate both at low power levels and at low voltages. In contrast to magnetic or thermally actuated devices, electrostatically actuated switches inherently operate at very low power levels, and are relatively simple to fabricate.
The microrelay performs a purely electronic function. We have fabricated two types of devices. The microrelay is a four terminal device as shown in FIG. 1a. Two terminals are used for actuation while the other two are switched. A second configuration is a three terminal device that we call a microswitch, shown in FIG. 1b. In either case, an electrostatic field applied between the beam (source) and the gate actuates the device. Switch closure shorts the beam tip to its counter electrode(s) thereby electrically connecting contacts a and b in the microrelay (or the source and drain in the microswitch). (The key difference between the microswitch and the microrelay in the terminology used herein is the presence or absence of electrical isolation between the actuator (the main part of the cantilever beam) and the contacts. This is independent of the number of contacts, and we have made switches with anywhere from 1 to at least 64 contacts.)
In previous publications, we have described the design, fabrication, and preliminary electrical characteristics of electrostatically-actuated, surface-micromachined, micromechanical switches and relays (P. M. Zavracky, et al., Microelectromechanical Systems, Ibid.; S. Majumder, P. M. Zavracky, N. E. McGruer, “Electrostatically Actuated Micromechanical Switches,” J. Vac. Sci. Tech. A, vol. 15, p. 1246, 1997; S. Majumder, N. E. McCruer, P. M. Zavracky, G. G. Adams, R. H. Morrison, and J. Krim, “Measurement and Modeling of Surface Micromachined, Electrostatically Actuated, Microswitches,” International Conference on Solid-State Sensors and Actuators, Digest of Technical Papers, Vol. 2, pp. 1145-1148, 1997; and S. Majumder, N. E. McGruer, P. M. Zavracky, R. H. Morrison, G. G. Adams, and J. Krim, “Contact Resistance Performance of Electrostatically Actuated Microswitches,” American Vacuum Society, 44th National Symposium Abstracts, p. 161, 1997). An SEM micrograph of such a microswitch is shown in FIG. 2. (In FIG. 2 the contacts are part of the beam—not isolated—and so it is a microswitch.) These switches are capable of over 1×109 switching cycles at low currents (4 mA) and at least 1×106 switching cycles at 100 mA. The anchored end (source) is on the right, and the contacts are under the cantilever beam to the left of the center of the micrograph.
These devices typically have threshold voltages for contact closure of 50 to 60 V, although we have produced many switches with threshold voltages of 20 to 30 V and a few low-contact-force switches that have operated at voltages as low as 6 V. Switching times are a few microseconds and switch lifetimes can be in excess of 1×109 cycles.
The microrelay has obvious advantages over conventional relays in being smaller and consuming less power. However, what is most attractive is that the microrelay can be integrated with other devices on a single die. Micromachined relays can be fabricated in large numbers on a single die which may contain other electronic devices. The lack of high temperature steps in the fabrication process described here means that the relays can be included as post-process additions to a conventional integrated circuit. Complex switching arrays and devices designed to handle high frequency signals with low insertion loss are natural extensions of the work described here.